It is makrūh (disliked) to single out Saturday for fasting. This is because it was narrated from ‘Abdullah Ibn Busr who narrated from his sister that Allāh’s Messenger said, “Do not fast Saturday unless it is a fast that Allāh has made an obligation for you. So, even if you can only find a grape skin or a piece of wood from a tree, you are to chew that.”

[This ḥadīth was graded as ṣaḥīḥ (authentic) by Al-Shaykh Al-Albānī in Al-Irwā`]